1. Field
One or more embodiments of the present invention relate to an enhancement of an acoustic signal, and more particularly, to a method, medium and apparatus for enhancing a bass signal using psychoacoustics and a masking effect.
2. Description of the Related Art
Speakers are electromechanical-acoustic devices that convert an electrical signal from an amplifier into sound. The audible sound is created by generating longitudinal waves in the air using a vibrating diaphragm, typically called a driver, or speaker driver. Speakers only reproduce a signal within a predetermined frequency range due to the structural characteristics of the speakers. That is, speakers can only reproduce a signal corresponding to a fixed frequency reproduction band from an electrical signal without distortions. The minimum reproduction frequency corresponding to the frequency reproduction band means the lowest frequency at which sound can be reproduced without distortion.
Thus, in order to reproduce a low frequency, or bass signal, speakers must be designed so that the minimum reproduction frequency is low, and in order to lower the minimum reproduction frequency of speakers, speakers must have a large-diameter driver and a relatively large cabinet volume to assure a sufficient vibrating depth.
However, due to a trend towards light, thin, and miniaturized electronic products, the size of speakers for generating sound from various acoustic products has gradually been miniaturized, and available space for speaker installation has also been reduced. Thus, for micro speakers used in mobile phones, portable multimedia devices, and headphones, speakers are only available whose maximum bass reproduction is in the hundreds of Hz, due to the limitation in the size of the micro speakers. However, since the human-audible band is conventionally 20 Hz to 20,000 Hz, a non-reproducible audible band exists for typical micro speakers corresponding to 20 Hz to hundreds of Hz, or the bass reproduction limit of the speakers. The structural limitations of these micro speakers cause the listener to hear only a relatively plain sound in which a deep, rich bass signal is not included.
In order to improve this problem of micro speakers, a technique of representing a bass signal using psychoacoustics has been developed. Psychoacoustics deals with the kind of psychological effects that sounds in different time, space, and frequency induce on a human. Psychoacoustics will now be described in detail.
FIG. 1 is a graph for describing a psychoacoustic bass perception effect using harmonics.
The phenomenon of representing bass signals using harmonics is called a virtual pitch or a missing fundamental frequency in psychoacoustics as described below. In FIG. 1, since a signal having a frequency f has a lower frequency than the minimum reproduction frequency of the speaker, the speaker does not reproduce the signal. The minimum reproduction frequency of the speaker denotes the lowest frequency at which sound can be reproduced by the speaker without distortion within the frequency reproduction band, i.e., a frequency corresponding to a point of which a slope is ‘0’ in the graph illustrated in FIG. 1. Since the speaker does not reproduce the signal having the frequency f, an audience cannot perceive the signal having the frequency f. However, if a sound generated by properly attenuating the harmonics, such as 2f, 3f, 4f, 5f, . . . , nf which are generated using the frequency f as a fundamental frequency, is provided to the audience, the audience can perceive the signal having the fundamental frequency f from the harmonics having n times the fundamental frequency f (n is a positive integer equal to or greater than 2). This phenomenon is called a virtual pitch or a missing fundamental frequency.
A psychoacoustic technique of representing bass signals using harmonics has been used for musical instruments, such as pipe organs, for many years. In addition, a technique of hearing bass signals using the psychoacoustic method is disclosed in an issued US patent. However, in the issued US patent, since only a portion, from which bass signals are perceived, is analogized from conventional acoustics and implemented using only an electrical circuit or algorithm, the sound quality, which is an important element of sound reproduction, is relatively poor.
The reason sound quality is relatively poor in the conventional psychoacoustic technique as described above will be described herein in detail.
FIG. 2 is a graph illustrating a frequency response curve and a Total Harmonic Distortion (THD) curve measured from a micro speaker.
THD denotes a ratio of harmonic components to a fundamental frequency and the harmonic components and is represented by Equation 1.THD=TotalDistortion/Total=√{square root over (a22+a32+a42+ . . . +an2)}/√{square root over (a12+a22+a32+a42 . . . +an2)}  Equation 1
Here, a1, denotes an amplitude of the fundamental frequency, a2 denotes an amplitude of a second harmonic, a3 denotes an amplitude of a third harmonic, and an denotes an amplitude of an nth harmonic.
If the THD is high, it may mean that a relatively large proportion of noise is mixed in with the audio, and thus sound quality is poor. That is, it can be seen using Equation 1 that the greater the number of harmonic components, the poorer the sound quality.
As illustrated in FIG. 2, a THD value varies according to frequency. Hence, the THD value is generally very low in the frequency reproduction band, however, the THD value is relatively high in a specific band (e.g., the A band of FIG. 2) of the frequency reproduction band and thus, sound quality is relatively poor in the conventional psychoacoustic method. Also, since a method of enhancing a bass signal using psychoacoustics allows a human being to perceive a signal having the fundamental frequency using the harmonic components, many harmonic components generated by setting a sound to be heard as the fundamental frequency exist. That is, in the conventional method of enhancing a bass signal using psychoacoustics, the sound quality is poor since a relatively high THD value is inevitable.